#617200 - 28/12/0607:18 PMRe: Know that if you do not post in this thread today the human race may cease to exist

Anonymous
Unregistered

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Originally posted by NY Madman:

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Originally posted by XPLORx4:[b]But your hang-up is how motion through the fluid is achieved, right?

No, not at all.

I've been discussing this hypothetical scenario. In the scenario we have NO MOTION through the fluid of air.

In the scenario we only have the thrust from the engines.[/b]

Where in here does it state there is no motion?

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Originally posted by Hawk:How about this.....lets see if this can get to the ALR:

A plane is standing on a runway that can move (like a giant conveyorbelt). This conveyor has a control system that tracks the plane'sspeed and tunes the speed of the conveyor to be exactly the same (butin the opposite direction).

A plane is standing on a runway that can move (like a giant conveyor belt). This conveyor has a control system that tracks the plane's speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). There's also a giant fan at the far end of the runway that blows a headwind at the plane in exactly the same amount of thrust the engines provide.

Will the plane be able to take off?

The answer to this is yes, but it won't have a "viable flight" because the plane won't be able to make any forward progress; the giant fan is always blowing a headwind that prevents the engine thrust from moving the plane forward.

This'll be a long post (cause I love Physics)Let's go back to the original hypothetical situation:

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A plane is standing on a runway that can move (like a giant conveyorbelt). This conveyor has a control system that tracks the plane's speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction).

Will the plane be able to take off?

A lot of people have gotten hung up on the conveyorbelt motion vs the wheels motion debate.Howver, the hypothetical scenario doesn't what kind of plane it is, or if it has wheels or pontoons.

In any event, the type of landing gear does not matter, because the landing gear is not reponsible for any propulsion in the airplane whatsoever. The landing gear only allows for the movement and steering of the plane on land/water.

Take your old Radio Flyer Wagon, for example. The wheels of your RFW allowed you to steer the wagon, but they did not make the wagon go - either gravity or someone(thing) pulling it created the motion. The wheel rotation just allows the wagon to move easily.

The conveyor belt speed vs wheel speed is a red herring - You could easily assume the plane is a pontoon style seaplane trying to take off using a water treadmill.

I gather from what is said in the hypothetical sscenario that the plane will remain in one place relative to the ground at any engine/plane speed because of the conveyor moving in the exact same speed in an oposite direction. That means at any Plane/conveyor belt speed, the difference in velocity between the airplane's wings and the ground will always be 0 mph.So the Plane is moving, the runway conveyor is moving, and the assumption is that the body of the plane does not move relative to the ground.

And the relative motion between an airplanes wings and the ground is irrelavent to a plane achieving lift.

What we have to remember is that we are talking about a plane, in which the thrust created by the engine is created by the engine moving the air. The airplane wing in essense does not care what the relative speed of the wing is to the ground, but rather what the relative speed of the wing is to the air. That's what is going to generate the lift required for takeoff...

Now their are other factors for an airplane to takeoff, but since we will assume that the plane would have no problems taking off under normal conditions (the "treadmill" is not what I would call a normal condition)

Lift is generated by the difference in velocity between a solid object (the wing) and a fluid (air - a gas is a fluid). There must be motion between the object and the fluid: no motion, no lift. It makes no difference whether the object moves through a static fluid, or the fluid moves past a static solid object.

Now while the airplane appears to be staying in one place relative to the ground because of the "treadmill" under the plane, we know that air movement must be taking place. We know this because the motion of an airplane is created by the engine moving the air. Without any air movement, the plane and treadmill would remain stationary.

Keep in mind we aqre dealing with velocity (directional speed).Adding 200mph east to 200mph west = 0 mph overall movement, even though the two moving objects are moving at 200 mph be themselves.

Because the airplan wings are affixed to the airplane, the wing and plane speed are the same.

Assuming the airspeed is 0mph...

So if the treadmill is is moving at 0mph, that means the the airplane must also be moving at 0mph, and that means the difference in velocity between the wings and air are both 0 mph - no lift possible

Now if the treadmill is moving at 200mph (west), that means the the plane must also be moving at 200 mph (east) - and while the difference in velocity between the Plane and treadmill remains 0 mph, the difference in velocity between the wing and air is 200 mph.

For the treadmill to reach speeds of 200mph, the plane has to be travelling 200mph (in the opposite direction) and while the ground is not moving in relation to the plane, the air is...since the air has to be moving for an airplane to move (engines) and thus for the treadmill to move. Thus this means the assumption that the plane remains stationary on the treadmill is false.

So yes, the Plane will be able to take off on the treadmill, assuming the plane could take off under normal conditons to begin with.

As for the bullets - both bullets (ignoring air resistance) would land at the same time. Both bullets are subject to the force of gravity (~9.8 m/s^2 - with slight variances based on locations: due to differing densities of the underlying crust, that the earth is not perfectly spherical, and the centripetal force from the motion of the earth's rotation on its axis varies at different latitudes.) So at the moment of being fired or dropped, the bullet begins to fall towards the earth with an acceleration of about 9.8 m/s^2.

The fastest speed of a fired bullet is ~1500m/s (~467.3ft/s) - bullet speeds depend on the type of bullet. The time it takes a bullet to travel 32.8 meters (10 feet) at 1500m/s (476.3ft/s) is 0.021867 seconds. In that time, the bullet will have fallen towards the earth approximately 0.002344 meters (0.000714 ft)...that is pretty much inisignificant to the human eye, which makes it appear as if the bullet is defying gravity.If the bullets are fired and dropped at a hieght of 19.6 meters (5.97ft), they will both hit the ground at the exact same time (2 seconds later), but the dropped bullet will be directly below the spot where it was dropped, while the fired bullet will be found ~3000 meters away (914.36ft) maximum (again, ignoring air resistance)

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JeffreyI'm just trying to put my tires on the rocks of life.

What's MOST interesting about this thread is its been able to illustrate those able to think, and those unable to think. (Well)

It seems the major mistake is thinking a plane moves like an Xterra.

If we remove that one obstacle, maybe 90% of the "Can't Fly" camp will see the light.

The Plane = Xterra confusion renders them thinking that the foward thrust of the engine won't make the plane go forward.

All the rollerskate, skateboard, ice, etc...analogies are lost on them...

The next concept is that the tires will move foward at a different actual speed than their rotation.

What the "Can't flies" are imagining is an Xterra on this belt, trying to go forward...and its just sitting there, spinning it's tires faster and faster as the belt matches its speed.

Of course, the Xterra can ONLY move if pushed by its tires...so the X = plane crowd are dead on about the X part.

The next BIG concept is that the plane's TIRE'S ARE NOT PUSHING IT.

The Engines thrust is pushing it...and the engines have NO idea what's going on down there...they just push the plane forward.

If we say that the ground speed of the plane and the air speed of the plane are the same based upon GPS coordinates during the take off...it might help.

OK - I'm firing up the engines...we have thrust....it pushes the plane forwards, to new GPS coordinates further down the accellerating conveyor, which is rolling under the plane at the plane's speed...

So the plane is moving across the gps coordinates at say 20 mph, and the belt is going towards it at 20 mph...so the tires are spinning at 40 mph....the plane is NOT where it started by GPS...its further down the belt's length.

By the time the plane's speed across the gps coordinates reaches it's take off speed, enough lift will be created by the air rushing over the wings, and the plane will be way down the runway/end of the conveyor belt...and the plane will be able to take off.

Originally posted by TJ:What's MOST interesting about this thread is its been able to illustrate those able to think, and those unable to think. (Well)

or, in some case *cough*Madman*cough* at all...

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Originally posted by TJ:I can't BELIEVE how many people this doesn't make sense to.

Well, it's similar to the discussion we had at ECXC, in that you have some people who are arguing about something they really don't understand (but think they are experts in).Of course, at some point and time, we are all guilty of that. It's realizing and admitting it which is difficult

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JeffreyI'm just trying to put my tires on the rocks of life.

Rock -- I understand exactly what you are saying. It was a good post...(until your next post with your obvious jab).

I do think we are not all looking at the scenario the same way. Some have made comments about the wheels and friction. I wasn't even considering that because that is irrelevant to the given scenario.

Considering that this is a hypothetical situation, I'm not sure there is a correct answer. I think we are all incorporating some assumptions into our answers.

I just did a little internet searching on this scenario and I was surprised to see that it has been discussed in many places on the web. This is the first time I've ever seen this question posed. Some sites have extremely long threads regarding this topic. I haven't found any place that seems to provide a consensus even among people who claim to be engineers and scientists.

I have seen many people start calling each other names and attacking each other over a hypothetical situation. Maybe that is why someone here thought this thread would go to the ALR. It's good to see that this discussion has been civil for the most part.

Does the question or scenario have enough information to generate a correct answer? I don't know. Maybe not. There are very valid points to be made for and against the flight of the aircraft in this hypothetical scenario.

#617212 - 28/12/0609:46 PMRe: Know that if you do not post in this thread today the human race may cease to exist

Anonymous
Unregistered

Madman, check out my last post on page 8. If your theory was correct and the conveyor (treadmill) could apply force to counteract the aircraft, the little balsa prop would not take off. Yet it does. Try it yourself.

Originally posted by NY Madman:Rock -- I understand exactly what you are saying. It was a good post...(until your next post with your obvious jab).

Aww, it's not like you've never taken a "jab" at anyone before, now is it? And the "jab" doesn't discount the information or the point I was making.I'm not sure how the quality of that post is reduced because of a joke made in another...

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Originally posted by NY Madman:Some have made comments about the wheels and friction. I wasn't even considering that because that is irrelevant to the given scenario.

Then I'm having trouble understanding why you posted this on the second page of the thread...

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Originally posted by NY Madman:

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Originally posted by TJ:

The lift is from air speed, which has zero to do with ground speed or tire speed.

You're wrong.

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JeffreyI'm just trying to put my tires on the rocks of life.

Its funny - the same arguments are going on all over the web it seems...and, there's people here who read the arguments there, and here, and depite the amazing similarities, merely conclude that there's no consensus...w/o absorbing any of the concepts presented.

I'm adding a concept btw:

I CAN breath if I'm on a tread mill, it does not prevent airflow into my lungs.

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A jet engine can draw air into it, as can a propeller plane draw air towards the prop...

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Maybe people are thinking of the conveyor belt given as more of an entire planet, with its own atmosphere and gravity or something, rather than as the described big tread mill?

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As the conveyor matches the plane's speed...if the plane is at rest, so is the belt.

I think for the example we have to assume its as given, an exact match...not a overshoot/correct/undershoot kind of thing.

I also think some of the "CF's" just can't get past the plane not = X thing...

Why would any one think air, wind, rain, snow, hail, butterflies, etc...would not blow about even if there's a tread mill under them?

If I go outside on my tread mill, and start walking at my usual 4 mph, and its raining...I get wet.

If you push me from behind, I will fall forward...if you strap a ACME Jet Pack to my back...I will fly off the tread mill...

If the tread mill spins at 60 mph...that jet pack will still send me flying off it.

Take a Rocket of sufficient size to propel the X without the engine and strap it to the roof.

Operate the engine to gain let's say 90MPH or near the limiter, Perfectly safe on the dyno right. Disconnect the straps and fire the rocket.

Ok, assuming the rocket goes straight and doesn't sprial our of control:the Xterra stays on the Dyno while the trocket goes flying forward because, after disconnecting the straps, the rocket is no longer attached to the Xterra...so the Xterra is thus unaffected by the rocket propulsion. Although the pressure from the air movement from the rocket could possibly cause the Xterra to come off the dyno, but not neccesarily.

If the rocket spirals out of control - well, what the Xterra does depends on if the rocket flys back into the Xterra. If so, I would guess the Xterra comes off the dyno.

In either case, I would want to be really, really, really far away from wherever people are planning on doing this test

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JeffreyI'm just trying to put my tires on the rocks of life.

Originally posted by NY Madman:[b]Rock -- I understand exactly what you are saying. It was a good post...(until your next post with your obvious jab).

Aww, it's not like you've never taken a "jab" at anyone before, now is it? And the "jab" doesn't discount the information or the point I was making.I'm not sure how the quality of that post is reduced because of a joke made in another... [/b]

I never said the quality of the post was reduced by your jab. That's an assumption on your part. Forget about it. It makes no difference. It's just routine ball busting and you are correct that I also do it occasionally.

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Then I'm having trouble understanding why you posted this on the second page of the thread...

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Originally posted by NY Madman:

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Originally posted by TJ:

[b]The lift is from air speed, which has zero to do with ground speed or tire speed.

You're wrong.

[/b]

I wasn't referring to the tires or anything related to the wheels in that particular comment.

I also believe that I was wrong for saying definitively that the aircraft in this scenario would not obtain flight.

I do however see a problem with the scenario. I can't say it would or would not take flight at this point.

OK, based on the original plane/conveyor statement, we find that the conveyor speed is dictated by the plane's speed. How is the plane's speed measured so that the conveyor can move at that speed, too?

1. The plane's speed is determined by a sensor on a wheel and the plane's speed is assumed to be based on the rotational speed of the wheel (like a car speedometer). As the forward thrust of the plane's engine(s) start moving the plane forward, the wheels start rotating.

At the first feedback cycle, the conveyor system detects a 1 mph rotational speed of the wheel, so the conveyor starts moving in the opposite direction at 1 mph. What's this? Now the wheel is rotating at 2 mph. OK, the conveyor does the same. Now the wheel appears to be moving at 4 mph, so the conveyor does the same.

Do the math. Very quickly, the conveyor speed will approach infinity. So then, the question becomes - based on the conveyor's measurement and feedback cycle, how fast will the plane's tires blow up due to rotating beyond their design limits.

2. The plane's speed is determined based upon measurements against a fixed point on the ground (not on the conveyor). When the plane is moving 10 mph relative to the fixed point, the conveyor also moves 10 mph in the opposite direction. The plane is moving through the air at 10 mph and the wheels are rotating as if it were traveling at 20 mph.

Let's say the plane can rotate and lift off the ground moving at a speed of 100 mph (air speed if no wind and also ground speed compared to the imaginary fixed point). The plane will lift off. Just before leaving the conveyor, the tires will be rotating twice as fast than if it were lifting off from a fixed runway.

In summary, the moving conveyor does nothing to affect the air moving across the plane's wings, only to affect the rotational speed of the plane's tires. The plane moves from thrust imparted upon the air by the propellors or turbines to move the body of the plane forward regardless of wheel speed and will eventually take off. (Disregarding, of course, the natural physical effects of wheels potentially spinning at some speed at which they were not designed to handle).

Edited to add: Now that this problem is solved, here's another one to consider: Using common mathematical equations, prove that 1 = -1.